In order to prevent deterioration of a waveform of a data signal, a device such as a data relay and the like extracts a clock signal component from an input data signal, reads the data signal at a level transition timing of the clock signal, and outputs the read data signal.
As the performance required for this kind of device, the degree of improvement in jitter (i.e., phase fluctuation) of the input data signal, that is, the ability to suppress the transfer of jitter is defined as a jitter transfer characteristic.
Conventionally, an apparatus 10 for measuring a jitter transfer characteristic having the configuration shown in FIG. 7 has been used to measure the jitter transfer characteristic.
The apparatus 10 for measuring the jitter transfer characteristic includes a jitter generator 11 which outputs a clock signal C phase-modulated by an input modulation signal M, a modulation signal generator 12 which generates a sinusoidal modulation signal M having a known amplitude m and a frequency f and outputs the sinusoidal modulation signal M to the jitter generator 11, and a data signal generator 13 which generates a data signal D (e.g., a pseudo-random signal) synchronized with the clock signal C output from the jitter generator 11 and provides the data signal D to a measurement object 1 such as a relay and the like. Meanwhile, the modulation signal generator 12 is configured to change the frequency f of the modulation signal M according to an instruction of an operation processor 18, which will be described later.
The measurement object 1 receiving the data signal D performs a reading process on the data signal D and outputs the read data signal D′.
A clock recovery unit 15 receives the read data signal D′ from the measurement object 1, recovers a clock signal component C′ from the data signal D′, and outputs the clock signal component C′ to a phase detector 16.
The phase detector 16 detects the phase of the clock signal component C′ and provides a signal M′ obtained by the phase detection to an amplitude detector 17. The amplitude detector 17 calculates an amplitude m′ by detecting the signal M′ and provides the amplitude m′ to the operation processor 18.
The operation processor 18 calculates the ratio m′/m of the amplitude m′ obtained by the amplitude detector 17 and the known amplitude m of the modulation signal M and calculates the amount of jitter transfer Tj(f) of the measurement object 1 at the jitter frequency f as a logarithm of the ratio. Upon completion of the calculation, the operation processor 18 converts the frequency f of the modulation signal M into another frequency and repeats the process for calculating the amount of jitter transfer Tj(f), thus obtaining a jitter transfer characteristic F indicating how the amount of jitter transfer is changed within a desired frequency range as shown in FIG. 6.
It is possible to determine the amount of jitter transfer (the amount of suppression) of the measurement object 1 at each jitter frequency from the thus obtained jitter transfer characteristic F.
Meanwhile, the apparatus for measuring the jitter transfer characteristic with the above-described configuration is disclosed, for example, in Japanese Patent Publication No. 1996-220163.
However, the above-described conventional apparatus for measuring the jitter transfer characteristic requires an averaging process in consideration of the influence of noise when calculating an amplitude V′ of the detected modulation signal M. Therefore, the conventional apparatus requires a long time to obtain the amplitudes V′ as many as the number of points of the required jitter frequencies when the measurement is made at each jitter frequency point in the above-described manner, which is inefficient.